Re: [RFC][PATCH] mm: ifdef out VM_BUG_ON check on PREEMPT_RT_FULL

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 1 Jun 2015 15:00:47 -0400
Johannes Weiner <hannes@cmpxchg.org> wrote:

> Andrew's suggestion makes sense, we can probably just delete the check
> as long as we keep the comment.
> 
> That being said, I think it's a little weird that this doesn't work:
> 
> spin_lock_irq()
> BUG_ON(!irqs_disabled())
> spin_unlock_irq()
> 
> I'd expect that if you change the meaning of spin_lock_irq() from
> "mask hardware interrupts" to "disable preemption by tophalf", you
> would update the irqs_disabled() macro to match.  Most people using
> this check probably don't care about the hardware state, only that
> they don't get preempted by an interfering interrupt handler, no?

The thing is, in -rt, there's no state to check if a spin_lock_irq()
was done. Adding that would add overhead to the rt_mutexes without much
gain.

The fast path of spin_lock_irq() in -rt looks like this:

	migrate_disable();
	rt_mutex_cmpxchg(lock, NULL, current);

Now, the migrate_disable() is more like preempt disable.

Although, maybe we could have -rt change irq_disabled() just check
that, and add a raw_irq_disabled() for when we need to make sure
interrupts are really off.

-- Steve


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